How snow forms
Hi everyone! Today, we continue our series of winter articles, and finally tell you how snow forms!
The three states of water
Water on Earth can be a liquid, a solid, or a gas. It can even be many of these forms, at the same time and in the same place! Everyone who has been to temperate and polar latitudes has seen these forms together on the surface of the Earth.
The photo shows water in all its states: liquid, solid, and gas (which we cannot see, but it is there). Photo: Phil Reaves / Unslash
In the atmosphere, water also comes in all three states. If the temperature of a cloud is between −20 °C and −10 °C, it will have both liquid water droplets and ice crystals. And water vapor (gas) is always present in the air. That’s precisely what we need for it to snow.
Valerya Milovanova / Windy.app
The composition of a cloud depends on its temperature. Up to about −10°C, it consists only of water droplets; between −20°C and −10°C, it consists of water droplets and ice crystals; below −20°C, it consists only of ice crystals.
A snowflake is born
Many people remember the school experiment with freezing distilled (unadulterated) water. Even after several hours in the freezer, it will remain liquid. But if you put an object into it or shake it, it will instantly become ice.
Something similar happens in clouds. There, at sub-zero temperatures, droplets of liquid water, called supercooled water, fly around. They will freeze if they meet aerosols—tiny particles of dust or smoke, grains of sand, salt crystals, microorganisms, or emissions from industries. The result is an ice crystal—the basis of a future snowflake.
The snowflake grows up
Next, the tiny crystals begin to grow in size. This happens thanks to the Bergeron process—here’s how it works.
Most water droplets don’t freeze, but remain liquid. Water evaporates from them continuously—just like from a mug or a puddle on the pavement. Some of the vaporized water remains as gas, some of it precipitates back onto the drop, but some of it sticks to the ice crystals, and it can’t get away from them. Such is the cloud crime: ice crystals take moisture away from defenseless droplets.
Bergeron’s process: water evaporates from the surface of the droplets, and settles on snowflakes. Valerya Milovanova / Windy.app
Most commonly, these crystals grow into flat snowflakes with six rays, which we have all seen since childhood in pictures. This is due to structural characteristics of water molecules.
Snowfall
Snowflakes hang in the cloud for a while, gaining moisture and growing in size. The stronger the upward air currents and the more moisture in the cloud, the larger the snowflakes have time to become. And when the cloud can no longer hold them, the snowflakes will fall. It’s snowing!
If the temperature is not too low (above −5°C), snowflakes will clump together. The largest snowflake in history was almost 40 centimeters in diameter!
Snow in the middle of a clear sky
Many people have seen it snowing when the weather is cloudless. It’s not really snowfall, but the winter equivalent of fog. It is not heavy, and forms when warmer air saturated with water vapor comes into an area with very cold weather. The air will begin to cool, reach a state of saturation, and the water vapor will be released from it in the form of small snowflakes or ice needles. By the way, there is a beautiful phenomenon associated with ice needles—light pillars.
What’s wrong with artificial snow?
Fans of winter sports are familiar with artificial snow, which is used to cover ski tracks and ski slopes. It is applied to the slope with special snow cannons.
Now it is clear how such snow differs from real snow: snow guns are simply not able to recreate all the stages of snowflake growth in the atmosphere. Artificial snow is just frozen water droplets, they do not have the volume and structure that snowflakes have. Hence all the differences in their properties.
Bonus! Are snowflakes really never the same?
No, they aren’t!It is said that no two snowflakes in the world have ever fallen the same way. And the number of possible shapes is greater than the number of atoms in the universe. Snowflakes are so diverse that even a classification of their shapes is very difficult to make.
A fragment of a collection of snowflakes photographed by Wilson Bentley in the late 19th and early 20th centuries. Photo: Wilson Bentley / Wiki
There is only one small «but»: there are snowflakes of very similar simple shapes. For example, prism snowflakes, formed at very low humidity. At first glance they will look exactly the same, but even they will always have microscopic features.
Text: Eugenio Monti, a meteorologist and a climatologist
Cover photo: Craig Bradford / Unsplash
Read more:
What are glaciers and how do they work
What is polar night and where does it occur
